Because of the adoption of 'koine' as official language of the Macedonian kingdom, the local dialect had been limited to oral communication since the 4th century BC. During the Roman period the only traces of its existence were certain names which were still in use. As Christianity spread in Macedonia, however, these names were gradually replaced by Christian ones. The last remnants of the ancient Macedonian dialect disappeared in the 4th century AD.
The 'koine' which had replaced it (except in administration where Latin was also used) was greatly simplified compared to the 'koine' of the 4th century BC, due to dramatic changes that had already started in the 2nd century BC. In addition, 'koine' had gradually acquired dialectal features pertaining to the modern rather than the ancient dialects. Thus, in the 2nd to 3rd centuries AD the 'koine' spoken in Macedonia exhibited traits, such as the loss of word-final unstressed vowels, which characterise the Macedonian dialects to this day.
While the spoken language moved progressively further away from the classical norm, official language of the state remained the more conservative form of 'koine'. The prestige of the latter increased further when the Fathers of the Church adopted it for the writing of religious and proselytising texts. The result was a constantly increasing gap between the spoken and the written language, a feature of Modern Greek even in the 20th century.